The invention relates generally to radiographic detectors for diagnostic imaging and particularly to a method and system of forming a multi-layer detector array with improved saturation characteristics.
In radiographic systems, an X-ray source emits radiation (i.e., X-rays) towards a subject or object, such as a patient or luggage to be imaged. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The X-ray beams, after being attenuated by the subject or object, impinge upon an array of radiation detector elements of an electronic detector. The intensity of radiation beams reaching the detector is typically dependent on the attenuation and absorption of X-rays through the scanned subject or object. At the detector, a scintillator converts the X-ray radiation to lower energy optical photons that strike the detector elements. Each of the detector elements then produces a separate electrical signal indicative of the amount of X-ray radiation at the particular location of the element. The electrical signals are collected, digitized and transmitted to a data processing system for analysis and further processing to reconstruct an image.
Conventional CT imaging systems utilize detectors that convert radiographic energy into current signals that are integrated over a time period, then measured and ultimately digitized. A drawback of such detectors however is their inability to provide data or feedback as to the number and/or energy of photons detected. Such data could be used during image reconstruction to distinguish between different types of materials, a capability which is unavailable for images reconstructed by conventional CT systems that. In particular, in a conventional CT system, the detector is unable to provide energy discriminatory data or otherwise count the number and/or measure the energy of photons actually received by a given detector element or pixel. That is, the light emitted by the scintillator is a function of the number of X-rays impinged as well as the energy level of the X-rays. Under the charge integration operation mode, the photodiode is not capable of discriminating between the energy level and the photon count from the scintillation. For example, two scintillators may illuminate with equivalent intensity and provide equivalent output to their respective photodiodes. Yet, the number of X-rays received by each scintillator may be different as well as the X-rays' energy, but yield an equivalent light output.
In attempts to design scintillator based detectors capable of photon counting and energy discrimination, detectors constructed from scintillators coupled to either avalanche photodiodes (APDs) or photomultipliers have also been employed. However, there are varying problems that limit the use of these detectors. APDs require additional gain to enable photon counting, but suffer from added gain-instability noise, temperature sensitivity, and other reliability issues. Photomultiplier tubes are too large, mechanically fragile, and costly for high-resolution detectors covering large areas as used in CT. As such, photomultiplier tubes have been limited to use in PET or SPECT systems.
To overcome these shortcomings, energy discriminating, detectors capable of not only X-ray counting, but of also providing a measurement of the energy level of each X-ray detected have been employed in CT systems. In particular, direct conversion detectors encounter very high photon flux rates as with conventional CT systems. For high flux signals there is a possibility that multiple X-ray photons will deposit their charge in a time shorter than the response period of a single element. Hence, flux above a certain threshold may lead to detector non-linearity or saturation and loss of imaging information.